1. All abdominal gills ventral; gill lamellae slender and lanceolate-type; numerous gill fillaments radiating out from a central plate (Fig. 2.2.1); rare; large rivers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Raptoheptagenia cruentata
Abdominal gills lateral in position, at least on abdominal segments 4 to 6; lamellae usually broad; gill filaments variable but not as above; two or three caudal filaments . . . . . . . . . 2
2. With only two well developed caudal filaments (Fig. 2.2.2). . . . . . . . . . . . . . . . .Epeorus
With three well developed caudal filaments (Fig. 2.2.3). . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Gills on abdominal segment 1 and 7 enlarged and meet, or almost meet, beneath abdomen to form ventral disk (Fig. 2.2.4). . . . . . . . . . . . . . . . . . . . . . . Rhithrogena manifesta
Gills on abdominal segment 1 and 7 do not meet beneath abdomen and usually smaller
than intermediate pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Gills of abdominal segment 7 minute, not longer than posterolateral projections of segment 7; rare. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Macdunnoa perisimplex
Gills of abdominal segment 7 much larger than above. . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Gills on segment 7 reduced to slender filaments (Fig. 2.2.5); trachea absent or with few or no lateral branches (Fig. 2.2.6). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Gills on segment 7 similar to preceding pairs but smaller (Fig. 2.2.7); trachea of gill 7 with lateral branches (Fig. 2.2.8). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Gills on abdominal segments 1-6 with apex pointed (Fig. 2.2.9).. . . . . . . . . . . Stenacron
Gills on abdominal segments 1-6 with apex rounded or truncated . . . . . . . . . . . . . . . . . 6
7. Gills on abdominal segments 1-6 with apex rounded (Fig. 2.2.10). Note unique pattern on tergae (Fig. 2.11) and ventral sterna (Fig. 2.2.12). . . . . . . . . . . Stenonema femoratum
Gills on abdominal segments 1-6 with apex truncated (Fig. 2.13). . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stenonema (all other species)
Note: The recent DNA analysis of the Stenonema/Maccaffertium genus by Zembrzuski and Anderson (2018) has returned the genus name to Stenonema. This change has been accepted by Burian (2019) in the recent edition of Merritt, Cummins and Berg (2019). The genus remains as Maccaffertium except for Stenonema femoralus by Mayfly Central.
8. Gills on segment 7 with fibrilliform portion present and with numberous fibrils; claws without distinct denticles but with one basal tooth . . . . . . . . . . . . . . . . . . . Heptagenia
Gills on segment 7 without fibrilliform portion, claws with distinct denticles . . . . . . . . . . 9
9. Caudal filaments with well developed interfacing setae (Fig. 1.16); head less than or equal to the width of the pronotum (Fig. 2.17), head occasionally with a few scattered black spots near anterior margin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nixe
Caudal filaments without well developed interfacing setae (Fig. 1.18); head wider than pronotum, often with dark spots along anterior margin (Fig. 1.19). . . . . . . . .Leucrocuta
Key to Ohio Heptageniidae Nymph Genera
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